I've gotten this question a few times, so I decided to write up a small explanation of the overhead with some sample numbers based on common case. You can read it on my blog at http://netsekure.org/2010/03/tls-overhead/.

Summary from blog post:

The total overhead to establish a new TLS session comes to about 6.5k bytes on average.

The total overhead to resume an existing TLS session comes to about 330 bytes on average.